This invention relates, in general, to semiconductor devices, and more particularly, to current limiting semiconductor devices.
In some semiconductor applications, such as high power motor controllers, it is necessary to integrate high voltage devices with low voltage control logic. The low voltage control logic regulates the amount of power provided to the motor by the high voltage devices. One element commonly used to provide the interface between the high voltage circuit and the low voltage circuit, is a current limiter. Once the voltage of the high voltage circuit exceeds a saturation value, the current limiter generates a constant current to be used by the low voltage circuit.
Traditional current limiters have been formed using junction field effect transistors (JFETs). JFETs form depletion regions to control the current passing through an epitaxial layer. Typically, doped regions are formed at the top and bottom side of an epitaxial layer which are doped to the opposite conductivity as the epitaxial layer. A voltage potential is then used to form a depletion region which restricts the amount of current passing through the epitaxial layer. The performance of the JFET current limiter is determined by a variety of process parameters which are difficult to control. The resistivity of the epitaxial layer, the depth and doping concentration of the doped regions, and the distance between the doped regions can all dramatically affect the performance of the current limiter. In a high volume manufacturing operation, the inability to accurately control these parameters reduces the yield of the final product which in turn increases the manufacturing cost.
Due to the breakdown of a gate oxide or the junctions of a JFET, it is not possible to use JFETs in applications with voltages of 100 volts to 2500 volts. To provide the current limiting function in a high voltage application, circuits containing several elements have been used. In most cases, a sequence of transistors and resistors are used to reduce the voltage potential and provide the constant current value. Such circuits, however, require several additional device elements which increases the manufacturing cost.
By now it should be appreciated that it would be advantageous to provide a single semiconductor device that can be used as a current limiter and that does not require the amount of process control associated with JFETs. It would be of further advantage if the semiconductor device could be used in circuit applications of 100 volts to 2500 volts and did not require the use of a plurality of device elements to provide the current limiting function.